This invention relates to an acoustic telemetering device for determining the relative position of a submerged object with respect to a vehicle.
More particularly, the invention concerns an acoustic telemetering device for determining the position of a submerged towed object with respect to a towing ship. This towed object is, for example, a profiled body or "fish" connected through a cable to a ship and containing oceanographic apparatuses. It performs sub-surface evolutions at a certain distance from the bottom and contains, for example, an echo-sounder, a Doppler sonar or lateral sonar adapted to determine its trajectory with respect to the bottom of the water layer or to produce echograms of the bottom suface or of the underlying layers. Such a fish is described, for example, in the French Pat. No. 2 412 853.
When the ship moves over deep bottoms, the cable through which it is connected to the fish may reach very great lengths (several kilometers). Due to the drag of the towing cable, to the variations of the towing speed and to the currents, the position of the fish may vary substantially, thus making it difficult to know accurately its location.
A first known prior art method consists of laying on the water bottom an assembly of beacons located at known positions and to determine, with respect to the latter, both the position of the towing ship and that of the submerged fish. This method requires, in the case of a large evolution zone of the ship, the immersion of a great number of locating beacons and their frequent displacement, thus making its application difficult.
A second known method, which can be used independently of the extent of the navigation zones, consists of determining, by acoustic means, the relative position of the fish with respect to the ship, the position of the latter being determined by making use of locating systems at sea of another known type such as radio positioning systems.
A known device for carrying out this second method comprises acoustic wave sensors secured under the ship, near each other and an acoustic pulse emitter, placed on the fish. An acoustic telemering system is adapted to measure the propagation time of acoustic pulses transmitted from the emitter and respectively received by the sensors. The initial instant used as a reference for measuring the time intervals is that at which an emission order is transmitted, through the cable connecting the ship to the fish, for actuating the pulses emitter placed therein.
Another known device making use of the same method also comprises acoustic wave sensors secured under the ship in the vicinity of one another, an acoustic pulse emitter rigidly secured to the fish and also as a telemetering system measuring the time intervals for the acoustic pulses propagation between the emitter and the sensors. It differs from the preceding one in that the emission order is transmitted to the emitter located in the fish by means of an acoustic signal which propagates through water from the ship.
The known method, consisting of transmitting an acoustic signal from the fish and receiving it on the ship, suffers from disadvantages. The signal, very substantially weakened during its propagation from the object, is received in a high noise zone (noise of machines, wind, waves, noises generated by the surface navigation, etc . . . ) and the signal-to-noise ratio is low, being thus prejudicial to the accuracy of the effected telemetering measurements.
Besides, the acoustic wave sensors are generally secured under the hull of the ship at the immediate vicinity of one another, thus having a further unfavorable effect on the accuracy of the effected measurements.